Fluid coking residual oil



March l, 19610 J. F. MOSER, JR., ETAL 2,927,073

l FLUID COKING RESIDUAL OIL Filed oct. 7, 195s 2 sheets-sheet 1 JOHN F. MOSER JR. l

FRANK B. JOHNSON BY ATTORNEY 0 SOUTH LOUISIANA RESIDUUM A HAWKINS RESIDUUM v BILLINGS RESIDUUM lMaJ'Ch l, 1960 J. F. MOSER, JR., ETAL 2,927,073

FLUID COKING RESIDUAL. OIL

8 o o o o o o a oo e a Q w -f VISCOSITY, SSF AT 275F.

John F. Moser Jr.

Frank B. Johnson Inventors United States Patent O FLUID COKING RESIDUAL OIL John F. Moser, Jr., and Frank B. Johnson, Baton Rouge,

La., assgnors to Esso Research and Engineering Company, a corporation of Delaware This invention relates to the coking of residual oils and pertains more particularly to the coking of such oils in thepresence of hot lluidized solids.

It has previously been proposed to coke high boiling residual oils in the presence of finely divided solids maintained at a temperature sufficient to convert the oil into vapors and coke. According to one mode of operation, the stream of the solids is withdrawn from the coking vessel and cycled through an external heater in which sufficient heat is imparted to the solids to supply the necessary heat for the operation.

One of the limitations to the fluid type coking process is that the amount of oil which can be charged to the coking vessel per unit time for a given Weight of solids in the reactor must be carefully controlled.

For economic reasons, it i-s desirable to operate the coking vessel of given size at a maximum permissible feed rate. However, if the feed rate is too high, the solid particles in the bed lose their fluid characteristics due to the presence of unvaporized tarry matter present in the feed. This has become known as bed begging. The bogging of the bed is a particularly serious matter because when it happens, it is necessary to quickly interrupt the coking operation and to remove the solidified bed from the coking vessel. The latter is a laborious and time-consuming operation.

It is, therefore, important to be able to quickly detect potential bed bogging conditions so that appropriate action may be taken to avoid such disastrous consequences.

The main object of the present invention is to provide asimple and effective method of detecting potential bed bogging conditions.

It has been found that yfor a given set of coking conditions, such as coking temperature, solids residence time in the coking vessel, etc., the maximum permissible feed rate of oil which Will avoid begging the bed will depend upon the Conradson carbon content of the feed. The Conradson carbon content is a standard test for residuum oils described in numerous petroleum hand books, such as The Science o-f Petroleum, vol. 2, pp. 14-17 and 14-18. Briefly, it is the weight percent of solid residue remaining after heating a standard `sample under standard test conditions. It has been found that in a typical fluid coking operation in a test unit at a temperature of 1000" F., for example, the maximum permissible feed rate for 100% conversion which will avoid bogging of the bed is about 0.74 part by weight of feed per hour per part of weight of solids in the coking vessel when the feed has a Conradson c-arbon value of 18. When the feed has a Conradson carbon value of 23, the feed rate is about 0.58 and with a Conradson carbon value of 28, the feed rate is about 0.48.

For a given set of coking conditions, therefore, the feed rate can be controlled to avoid bed bogging by determining the Conradson carbon value of the feed. However, Conradson carbon determinations consume about onehalf hour and, therefore, cannot be used to quickly dey 2,927,073 Patented Mar. 1, 1960 tect rapid changes in the feed which may result from fluctuations in the crude distillation equipment from which the feed is derived.

It has been found that for a feed derived from any given crude source, there is a direct relationship between its viscosity Iand its Conradson carbon content. For example, a South Lousiana residuum having a Conradson carbon content of 18 will have a viscosity of about 70 S.S.F. (Saybolt seconds Furol) at 275 F., heavier residuum derived from the same source having a Conradson carbon content of 23 will have a viscosity of about S.S.F. at 275 F., and a still heavier residuum derived from the same source having a Conradson carbon content of 28 will have a viscosity of -about 250 S.S.F. at 275 F. A residuum derived from Hawkins crude having a Conradson carbon content of 24 will having a viscosity of about 350 S.S.F. at 275 F.

In accordance with the present invention, the relationship between the viscosity and the Conradson carbon value of various samples of the residuum to he coked is first established by simple laboratory methods and the feed rate of the oil to the coking unit expressed as the weight of oil treated per hour per weight of solids in the coking vessel is controlled by frequent or continuous viscosity determinations of the feed.

There are numerous types of continuous viscosity recorders capable of giving accurate and rapid viscosity `the operating conditions of the crude still and appropriate action can be taken to avoid bogging of the bed. Such corrective '..ction may involve reducing the feed rate, increasing the temperature of the coking vessel or by increasing the amount of solids contained in the coking vessel. The control may be manual or automatic.

Having described the general nature and objects of the invention, reference will now be made to the accompanying. drawing in which Figure 1 is a partly schematic and partly diagrammatic illustration of a fluid coking unit capable of utilizing the present invention; and Figure 2 is a curve showing the relationship between the viscosity of feed derived from certain crudes and the Conradson carbon value of such feeds.

Referring to the drawing, the reference character 1 designates the feed line in which the oil to be coked is introduced into the system. The feed for the process is preferably a hydrocarbon residuum such as atmospheric or vacuum bottoms from a crude distillation still, or it may be other types of heavy oils which are generally unsuitable for catalytic cracking, such as a heavy initial condensate from a catalytic cracking unit, commonly known as clarified oil.

The oil introduced through feed line 1 may be preheated to any desired temperature below active coking temperature for the oil. For example, ythe oil may be preheated to a temperature of from 50G-700 F. The

uidized condition by gases and vapors rising upwardly" therethrough.

The invention will be described using petroleum coke i The coke particles should begof a size range capable of maintaining good fluidity. For l.

as the circulating solids.

example, Athe average size of the individual particles in typicalV operation may,v range from 125 microns to 500 microns. It will be understood the particles grow during the process as a result of coke formed so that it is necessary to replace coarser coke with hner to maintain the desired size range. Someof the coke withdrawn may be ground and returned or grinding may be carried out in the operation. The density of the bed of iluidized solids in the coking vessel may range from 35 to 6() pounds per cubic foot. To obtain the desired turbulence in the bed, the minimum superficial velocity of the rising gases and vapors should be between .5 and 3 feet` per second and may be higher depending on the average particle size of the coke and other factors.

. The superficial velocity is the velocity the gases and vapors would travel in the absence of solids in the coking vessel. These gases and vapors will comprise all the gas introduced for stripping as later described and the vapors formed during the coking reaction.

The temperautre maintained in the coking vessel 2 may be between 900 and 1200 F. and sufficient to crack the heavy `oils into vapors and coke.

The intermediate section S of the reactor 2 above the lower feed injection line 6 may be tapered as indicated so that the velocity of the gases rising through the bed will be maintained more or less uniform throughout its depth despite the formation of additional gas and vapors in the upper section. The bottom section 9 of the reactor serves as a stripping well for removing vaporized products from the solids prior to circulating the solids to the burner vessel as hereinafter described. Stripping gas, such as steam, is introduced into the bottom of the stripping section through line 10 and branch lines 11, 12, and 13. A conduit 14 projects upwardly into the stripping section 9 for removing excess coke formed by the` process. Any large agglomerates which formed in the process` will be collected at the extreme bottom of the stripper section 9 and may be withdrawn therefrom through line 15.

A separate stream of coke particles is withdrawn from the stripping zone 9 and passed through -a transfer line 16 to the heater 17.

This transfer line is constructed and operated in a manner described in U.S. Patent 2,589,124, tiled May 1, 1950, and issued March l1, 1952. The transfer line is provided with a U-shaped conduit at the base which forms a seal for preventing passage of gases from the coking vessel to the heating vessel, or vice versa. The depth of the U-seal below the point of injection of the air to the heater is suflicient to develop a pressure greater than the normal fluctuations of the air pressure.

The solids, during their passage through the transfer line 16, are maintained in a fluidized condition. For example, aerating taps 18 may be provided at spaced points along the base of the U-bend to maintain the solids ina dense fluidized state. The amount ofv gas so introduced `is usually controlled to maintain the solids in a dense fluid state.

The transfer line 16 is provided with a riser section 19 which has its upper open end located within the heater vessel. Air is added to the riser through line 20. Additional air is introduced into the bottom of the heater 17 through line 21. The total amount of air added is controlled to burn enough coke to furnish the required heat for the process as before described. The amount of air added to the riser pipe 19 is regulated to control the flow of solids between the heater and the coking vessel. The remainingA air necessary is added to the bottom of the vessel 17.

As illustrated, the heater 17 contains a dense fluidized body of finely divided solids undergoing heating in the bottom section. Air is introduced into the bottom section" of the heater and passes upwardly through the bed at a velocity suicient to maintain the particles in a dense uidized state. The invention, however, is not restricted to any particular type of heater since anyftype of furnace could be used. ,i y.

--acaaovs e Y i. y

The heated particles overflow from heater 17 through an outlet line 22 which has an open end projecting up into the heater. The hot solids withdrawn from the heater 17 through outlet line 22 are transferred through a transfer line 2.3 and are reinjected into the reaction vessel 2 at an intermediate point in the coking vessel as illustrated. This transfer line is similar in construction to transfer line 15 and operates in a manner similar to U.S. Patent 2,589,124 previously mentioned. The vhot solids rom the heater 17 automatically overflow into the outlet. conduit 22 so that a constant level of fluidized particles' are maintained in theheater 17.

Hot combustion products from the heater vdischarge into a cyclone 24- which sewes to rem-ove particles cntrained in the combustion gases. The hot combustion products after passing through the cyclone 24 are removed through line 25. These gases may be subjected to further separation, such as by the use of additional cyclones, for final separation of the entrained powder.

Solids separated from the gases in the cyclone 24 arereturned to the dense bed through a dip pipe 26- which terminates below the level of the bed to provide an effective seal and prevent gases from blowing up through the dip pipe.

Vaporized hydrocarbons formed in lthe coking vessel,

2 are passed to a cyclone separator 27 located in the top of the vessel. Solids removed from the vapors in the cyclone are returned to the coking vessel throughthe dip pipe 28. The vaporized hydrocarbons are with:

drawn from the vessel through line 29 and subjected to further separation and purification as desired.

in carrying out the present invention, a continuous... sample of feed from line 1 is passed through continuousv viscosity indicator, recorder, or recordercontroller 30 maintained at constant temperature and the indicatedviscosity is used as a control to avo-id potential bed bogging. '3, Before starting the coking process, it is irst necessary to make preliminary tests to establish the relationship between the viscosity and the Conradson carbon value of the feed. The Conradson carbon value will determine the maximum permissible feed rate which will avoid bed bogging under the obtaining temperature conditions. t

For illustrative purposes, the relationship between the' viscosity of the residua derived from South Louisiana" and Billings crudes and the Conradson carbon value: is shown in the chart forming Figure 2 of the drawings. This figure shows the relationship between the Viscosity and the Conradson carbon value of the residuum. Itv

cannot be concluded that all residua regardless of source have the same relationship as shown on the curve.

simpleConradson carbon and viscosity determinations. Having determined this relationship and knowing'the maximum feed rate permissible for a feed having the Conradson carbon value so determined at the obtaining l. In a process of operating a fluid coking unit in which a stream of residual oil is introduced into a uidized mass of sub-divided solids maintained at a coking temperature in the range of 900 to 1200 F. and wherein the solids are maintained in a dense turbulent fluidized bedin the coking Zone by gases rising upwardly therethrough; the improved method of preventing loss of,

uidity of the bed which comprises.: determining the maximum permissible feed rate based on the Conradsoni carbon of the feed, determining the relationshipjbetwccn the viscosity and Conradson carbon ofthe feed, con--` However, this relationship can be readily determined by- If the viscosity of the feed should change l 5 -tnuously determining the viscosity of the feed passing into the coking zone and varying directly the severity of the coking operation with the viscosity of the oil when the viscosity of the feed changes a predetermined amount, the maximum feed rate employed being 0.74 part by weight of feed/part by weight solids/hour.

2. In a process deined in claim 1, the further' improvement which comprises reducing the feed rate to the coking zone when the viscosity of the feed increases a predetermined amount.

6 References Cited in the tile of this patent UNITED STATES PATENTS 2,345,272 Luhrs Mar. 28, 1944 5 2,485,315 Rex et al. Oct. 18, 1949 2,709,676 Krebs May 31, 1955 OTHER REFERENCES Handbook of Petroleum Asphalt and Natural Gas (1928 revision) p. 424. 10 Fuel Oil Manual (1951 pp. 64 and 65. 

1. IN A PROCESS OF OPERATING A FLUID COKING UNIT IN WHICH A STREAM OF RESIDUAL OIL IS INTRODUCED INTO A FLUIDIZED MASS OF SUB-DIVIDED SOLIDS MAINTAINED AT A COKING TEMPERATURE IN THE RANGE OF 900* TO 1200*F. AND WHEREIN THE SOLIDS ARE MAINTAINED IN A DENSE TURBULENT FLUIDIZED BED IN THE COKING ZONE BY GASES RISING UPWARDLY THERETHROUGH, THE IMPROVED METHOD OF PREVENTING LOSS OF FLUIDITY OF THE BED WHICH COMPRISES DETERMINING THE MAXIMUM PERMISSIBLE FEED RATE BASE ON THE CONRADSON CARBON OF THE FEED, DETERMINING THE RELATIONSHIP BETWEEN THE VISCOSITY AND CONRADSON CARBON OF THE FEED, CON- 